This invention relates generally to axial-flow fans, which may be used as automotive engine-cooling fans, among other uses.
Engine-cooling fans are used in automotive vehicles to move air through a set of heat exchangers which typically includes a radiator to cool an internal combustion engine, an air-conditioner condenser, and perhaps additional heat exchangers. These fans are generally positioned in a shroud which directs air between the heat exchangers and the fan and controls recirculation. Typically, these fans are powered by an electric motor which is supported by a plurality of arms which extend from a motor mount to the shroud.
The aerodynamic noise generated by these fans includes both broadband noise and acoustic tones. These tones are caused by time-varying forces on the blades, which are the response of the blades to upstream and downstream flow disturbances. The upstream disturbances are typically due to the non-axisymmetric nature of the shroud and heat exchangers, and the downstream disturbances are due to the motor-support arms and any other object which is close to the fan blades.
The spectrum of the noise generated by each blade in response to these flow disturbances consists of many harmonics of the shaft rotation rate. If the blades are evenly spaced, the spectrum of the noise generated by the entire fan consists only of harmonics of the blade rate—the product of the blade number and the shaft rate. Destructive interference cancels the harmonics between the blade rate harmonics, and constructive interference enhances the tones at the blade rate harmonics. These tones can be subjectively very annoying, and the designer often modifies the fan geometry to minimize this annoyance.
One way the designer can improve the subjective noise quality is to space the fan blades unevenly. In order to maintain good fan performance, the extent of the unevenness must be limited. But even with a modest amount of unevenness, the higher-order blade-rate harmonics of the fan spectrum can be significantly reduced. As the blade rate harmonics in the fan spectrum are reduced, the other shaft harmonics, which in the case of the evenly-spaced fan are non-existent, are increased. In other words, both the constructive and destructive tone cancellation is reduced if the blades are unevenly spaced. The result can be a fan with a noise characteristic which is subjectively less annoying than that of an evenly-spaced fan.
Because each blade of an unevenly-spaced fan sees a somewhat different inflow, and is required to develop a somewhat different amount of lift, the pitch and camber, and perhaps even the chord, of each blade might ideally be adjusted according to its position relative to the other blades. However, for reasonable amounts of unevenness, it is often possible to use blades with identical geometries. In fact, it is often observed that an evenly-spaced fan has the same performance as an unevenly-spaced fan which uses the same blade geometry.
One constraint on the design of a fan with unevenly-spaced blades is that the fan be balanced. Any imbalance in the fan can cause unsteady forces on the fan assembly which cause significant shaft-rate noise and vibration. Although a small amount of imbalance can be corrected by the addition or subtraction of weight (clips or balance balls) at particular locations, this is not practical when correcting a large amount of imbalance, such as that caused by improper blade spacing. Therefore, when calculating the desired position of fan blades, two of those blade positions must in general be determined by the balance requirement—one for balance around each of the transverse axes. If the blades are of identical design, such a strategy also guarantees that no couple imbalance will be caused by the uneven blade spacing.
Although a wide variety of blade spacing arrangements which assure balance is available to the designer of a fan with many blades, the designer of a fan with fewer blades has less choice. In particular, the spacing of the blades of a 4-blade fan has only one inter-blade spacing that can be selected arbitrarily. Once that space is selected, all other inter-blade spacings are determined by the balance requirement. A 3-blade fan is even more problematic, in that no unevenly-spaced blade arrangement is available that assures balance.
One solution to this problem is to always use at least 5 blades on a fan where some flexibility in blade spacing is desired. However, there are often aerodynamic advantages to the use of fewer blades. In particular, a lightly-loaded fan requires less blade solidity, and often benefits from using fewer blades rather than more blades with reduced blade area. A free-tip fan, in particular, benefits from the use of a small number of blades, since vortex-interaction noise is minimized by maximizing the distance between the fan blades.
There is therefore a need for fans which have the aerodynamic and noise advantages of small blade number, but the subjective noise advantage of uneven blade spacing.